This invention relates to a method of cutting hardened steel with high accuracy using a tool made of cBN sintered body.
As a method of preventing deterioration of roughness of a finished surface due to notch wear of a tool during cutting, unexamined Japanese patent publication 57-205002 discloses a cutting method in which the feed rate is changed within a predetermined range during the time period from the start to the end of cutting of a cutting surface for which uniform surface roughness is required. Unexamined Japanese patent publication 6-55301 discloses a control method for an NC cutting machine in which the feed rate of the tool cutting edge is changed within a predetermined range in each cutting cycle.
These methods are effective for the cutting of relatively soft workpieces, but will encounter the following problems in high-accuracy machining of a hardened steel having high hardness (HRc 50-65).
To be of high accuracy is to have a finished surface roughness of 3.2s (.div.3.2z) or less and to have a finished dimensional error of IT7 class or less. These two requirements have to be met. With the above method, if the workpiece is a hardened steel, dimensional accuracy does not stabilize.
Cutting of hardened steel is rather special compared with cutting of other materials. For example, the radial force that acts on the tool is large. This tendency becomes remarkable with the progression of wear of the tool.
If the feed rate is changed substantially under such circumstances, the radial force also changes significantly, thus decreasing the dimensional accuracy.
In the method of unexamined Japanese patent publication 57-205002, the feed rate is changed in the surface for which uniform surface roughness is required. Thus, dimensional changes tend to occur in the same surface due to changes in the radial force. In the method of unexamined Japanese patent publication 6-55301, the width of change in the feed rate is set not taking the radial force into consideration. That is, the feed rate is changed between the lower limit, which is the permissible limit of the extension of machining time due to the narrowing of width of the feed rate change, and the upper limit, which is the permissible limit of decrease in surface roughness due to the widening of the change width. Thus, during cutting of hardened steel, the radial force may change excessively. Also, no clear standard is determined for the timing of changing the feed rate. Thus, the surface roughness frequently does not improve as expected even if the feed rate is changed. Further, no attention is given to the shape of the tool used. Thus, the shape of the tool tends to be another cause of unstable dimensional accuracy.
An object of this invention is to provide a method of cutting hardened steel using a cBN sintered tool which can improve the surface roughness while maintaining high dimensional accuracy.